FIG. 4 shows a configuration of a conventional switching power supply device. The switching power supply device shown in this diagram is configured with: an input capacitor Cin, switches S1 to S4, a transformer T, diodes D21 and D22, an output winding L23, and an output capacitor Cout.
According to this switching power supply device, direct current (DC) input power at a predetermined voltage (Vin) applied between an input terminal TIN1 and an input terminal TIN2 is converted into alternating current (AC) power once by a switching operation of a full bridge circuit comprising the switches S1 to S4, and is then supplied to a primary winding L1 of a transformer T.
Due to the alternating current power having been supplied to the primary winding L1 of the transformer T, alternating current power at a predetermined voltage is induced in secondary windings L21 and L22 of the transformer T. This induced alternating current power is converted into DC power again by the main diodes D21 and D22, and is then externally outputted via the output winding L23 and an output terminal TOUT1 as direct current power at a predetermined voltage (Vout).
Incidentally, according to the switching power supply device mentioned above, when the direction of electric current flowing through the secondary windings L21 and L22 is switched, a surge shown in FIG. 5 caused by a leakage magnetic flux or the like between the primary side and the secondary side of the transformer T is generated on the cathode side of the main diodes D21 and D22.
Generally, this surge can be improved to some extent by providing a CR snubber circuit along with the main diodes D21 and D22. However, the basic principle of a CR snubber circuit is such that a resistance element consumes electric power and a surge is thereby absorbed. Therefore, it essentially causes an electric power loss, leading to reduced power supply efficiency. In particular, if the number of main diodes on the secondary side is increased, the number of CR snubber circuits to be provided along with the main diodes also increases. As a result, the electric power loss further increases.
As a technique for solving such a problem of the CR snubber circuit, there has been proposed a lossless snubber circuit that accumulates surge energy in a coil once and then discharges to an output side (refer to Patent Document 1).
Patent Document 1: Japanese Unexamined Patent Application, First Publication No. Hei 09-224374